Detection and Quantification of Gas Mixtures in Subterranean Formations

Abstract

Methods and systems are provided for quantifying contributions of gas mixtures in a reservoir compartment by way of isotopic analyses. Differing thermal maturities of the different gas mixtures allow estimation of the relative quantity of each gas mixture present in a total gas mixture. Thermal maturities may be estimated by reference to isotopic analyses of each contributing gas mixture and a commingled gas mixture resulting from commingling each of the individual source gas mixtures. This method may be carried out at various depths to determine relative contributions of each gas mixture to the total gas mixture as a function of wellbore depth. Advantages of certain embodiments include, but are not limited to, higher accuracies and ease of application as compared to conventional methods.

1 . A method for determining relative contributions of a plurality of gas mixtures to a reservoir compartment of a subterranean formation, the method comprising the steps of:
 (a) determining a first gas thermal maturity (R o     —     A ) of a first gas mixture, wherein the first gas mixture contributes to a commingled gas mixture in the reservoir compartment;   (b) determining a second gas thermal maturity (R o     —     B ) of a second gas mixture, wherein the second gas mixture contributes to a commingled gas mixture in the reservoir compartment;   (c) obtaining a plurality of samples of the commingled gas mixture at a plurality of depths, the commingled gas mixture at each depth characterized by a plurality of components, wherein the plurality of components comprises a plurality of carbon-based components, wherein each carbon-based component comprises a plurality of stable carbon isotopes;   (d) analyzing each of the samples from each depth to determine a stable carbon isotope value (δ 13 C) for two or more of the carbon-based components of each sample;   (e) determine a δ 13 C ratio of the stable carbon isotope value (δ 13 C) of a first carbon-based component to the stable carbon isotope value (δ 13 C) of a second carbon-based component, wherein the first carbon-based component is one of the two or more of the carbon-based components, and wherein the second carbon-based component is another of the two or more of the carbon-based components, wherein the δ 13 C ratio is determined at each of the plurality of depths;   (f) determining a commingled gas mixture thermal maturity (R o     —     m ) corresponding to the δ 13 C ratio determined in step (e), wherein determining the commingled gas mixture thermal maturity (R o     —     m ) is determined according to a known relationship of thermal maturity as a function of δ 13 C ratio; and   (g) determining the relative contribution of the second gas mixture to the reservoir compartment by evaluating the quantity (R o     —     m −R o     —     A )/(R o     —     B −R o     —     A ) or mathematical equivalent thereof to produce a second gas contribution (y).   
     
     
         2 . The method of  claim 1  wherein the known relationship is a linear relationship thermal maturity as a function of δ 13 C ratio; and wherein the step of determining the thermal maturity (R o     —     m ) (f) further comprises one or more steps selected from the group consisting of:
 (A) (i) plotting the linear relationship on a plot of stable carbon isotope values (δ 13 C) of the first carbon-based component versus stable carbon isotope values (δ 13 C) of the second carbon-based component to form a trend line on a first plot; (ii) for each δ 13 C ratio determined in step (e), plotting each δ 13 C ratio on the first plot to form a plurality of points on the first plot; (iii) projecting each point to the trend line to form a line normal to the trend line to form an intersection of the line with the trend line; and (iv) for each point, selecting the thermal maturity (R o     —     m ) at each intersection; and 
 (B) any mathematical equivalent of the combination of steps (i)-(iv). 
 
     
     
         3 . The method of  claim 2  wherein the first carbon-based component is methane and the second carbon-based component is ethane. 
     
     
         4 . The method of  claim 2  wherein the first carbon-based component is ethane and the second carbon-based component is propane. 
     
     
         5 . The method of  claim 2  wherein the first carbon-based component is methane and the second carbon-based component is propane. 
     
     
         6 . A method for determining relative contributions of a plurality of gas mixtures to a reservoir compartment of a subterranean formation, the method comprising the steps of:
 (a) determining a first gas thermal maturity (R o     —     A ) of a first gas mixture, wherein the first gas mixture contributes to a commingled gas mixture in the reservoir compartment;   (b) determining a second gas thermal maturity (R o     —     B ) of a second gas mixture, wherein the second gas mixture contributes to a commingled gas mixture in the reservoir compartment;   (c) obtaining a plurality of samples of the commingled gas mixture at a plurality of depths, the commingled gas mixture at each depth characterized by a plurality of components, wherein the plurality of components comprises a plurality of carbon-based components, wherein each carbon-based component comprises a plurality of stable carbon isotopes;   (d) analyzing each of the samples to determine a stable carbon isotope value (δ 13 C) of one of the carbon-based components of each sample;   (e) determining a commingled gas mixture thermal maturity (R o     —     m ) corresponding to the stable carbon isotope value (δ 13 C) determined in step (d) wherein determining the commingled gas mixture thermal maturity (R o     —     m ) is determined according to a known relationship of thermal maturity as a function of stable carbon isotope values for the carbon-based component; and   (f) determining the relative contribution of the second gas mixture to the reservoir compartment by evaluating the quantity (R o     —     m −R o     —     A )/(R o     —     B −R o     —     A ) or mathematical equivalent thereof to produce a second gas contribution (y).   
     
     
         7 . The method of  claim 6  wherein the second gas mixture is a miscible injectant gas. 
     
     
         8 . The method of  claim 6  wherein the known relationship is a known linear relationship. 
     
     
         9 . The method of  claim 6  further comprising determining the relative contribution of the first gas mixture to the reservoir compartment as the quantity (1−y), or (R o     —     m −R o     —     B )/(R o     —     A −R o     —     B ), or mathematical equivalent thereof to produce a first gas contribution (x). 
     
     
         10 . The method of  claim 6  wherein the samples are obtained at regular intervals of no more than about every 10 feet of wellbore depth. 
     
     
         11 . The method of  claim 6  wherein the samples are obtained at an interval frequency sufficient to minimize sample-to-sample variability to less than about 3 percent. 
     
     
         12 . The method of  claim 6  wherein steps (b) comprises obtaining a sample of the second gas mixture, analyzing one or more stable carbon isotope values of one or more carbon-based components of the second gas mixture, and determining the second gas thermal maturity (R o     —     B ) according to a known relationship thermal maturity as a function stable carbon isotope values. 
     
     
         13 . The method of  claim 6  wherein step (c) comprises the step of obtaining the samples from degassing of a mud receiving tank during drilling of the wellbore. 
     
     
         14 . The method of  claim 6  further comprising the step of analyzing a composition of each of the samples to determine a plurality of carbon-based component compositions as a function of depth. 
     
     
         15 . The method of  claim 6  wherein the second gas mixture is a miscible injectant gas. 
     
     
         16 . The method of  claim 6  wherein the thermal maturity of the first gas mixture is different from the thermal maturity of the second gas mixture. 
     
     
         17 . The method of  claim 6  wherein the first gas mixture was formed in a different source rock facies than the second gas mixture. 
     
     
         18 . The method of  claim 6  wherein the first gas mixture and the second gas mixture was formed in a first source rock, wherein the first gas mixture was charged into the first source rock at a time different than when the second gas mixture was first charged into the first source rock. 
     
     
         19 . The method of  claim 6  wherein the thermal maturity of the first gas mixture is different from the thermal maturity of the second gas mixture by at least about 5%. 
     
     
         20 . The method of  claim 6  wherein the mathematical equivalent comprises simultaneously solving the system of equations characterized by (x) (R o     —     A )+(y)(R o     —     B )=R o     —     m  and x+y=1, wherein x is the relative contribution of the first gas mixture, wherein y is the relative contribution of the second gas mixture, wherein R o     —     A  is the thermal maturity of the first gas mixture, wherein R o     —     B  is the thermal maturity of the second gas mixture, and wherein R o     —     m  is the thermal maturity of the mixed reservoir gases. 
     
     
         21 . The method of  claim 6  wherein the plurality of gases contains substantially only the first gas mixture and the second gas mixture. 
     
     
         22 . A method for determining relative contributions of a plurality of gas mixtures to a commingled gas mixture in a reservoir compartment of a subterranean formation, wherein the commingled gas mixture is characterized by a plurality of components, wherein the plurality of components comprises a plurality of carbon-based components, wherein each carbon-based component comprises a plurality of stable carbon isotopes, the method comprising the steps of:
 (a) receiving a first gas thermal maturity (R o     —     A ) of a first gas mixture, wherein the first gas mixture contributes to a commingled gas mixture in the reservoir compartment;   (b) receiving a second gas thermal maturity (R o     —     B ) of a second gas mixture, wherein the second gas mixture contributes to a commingled gas mixture in the reservoir compartment;   (c) receiving a stable carbon isotope value (δ 13 C) for two or more of the carbon-based components at each of the wellbore depths;   (d) determine a δ 13 C ratio of the stable carbon isotope value (δ 13 C) of a first carbon-based component to the stable carbon isotope value (δ 13 C) of a second carbon-based component, wherein the first carbon-based component is one of the two or more of the carbon-based components, and wherein the second carbon-based component is another of the two or more of the carbon-based components, wherein the δ 13 C ratio is determined at each of the plurality of depths;   (f) determining a commingled gas mixture thermal maturity (R o     —     m ) corresponding to the δ 13 C ratio determined in step (d), wherein determining the commingled gas mixture thermal maturity (R o     —     m ) is determined according to a known relationship of thermal maturity as a function of δ 13 C ratio; and   (g) determining the relative contribution of the second gas mixture to the reservoir compartment by evaluating the quantity (R o     —     m −R o     —     A )/(R o     —     B −R o     —     A ) or mathematical equivalent thereof to produce a second gas contribution (y).   
     
     
         23 . The method of  claim 22  wherein step (g) is determined by an information handling system, further comprising the step of outputting the second gas contribution (y) as a function of wellbore depth to user. 
     
     
         24 . The method of  claim 6  wherein the samples are obtained at regular intervals of no more than about every 5 feet of wellbore depth.